The Talili Pyroclastics eruption sequence: VEI 5 precursor to the seventh century CE caldera-forming event at Rabaul, Papua New Guinea

Abstract

Studies of the products of the activity precursory to caldera-forming events are essential in revealing how caldera systems evolve in the prelude to climactic eruptions. Here, we investigate the deposits formed by the Talili Pyroclastics eruption sequence, which preceded the youngest major eruptive event, the seventh century CE (1400 BP) Rabaul Pyroclastics VEI 6 caldera-forming eruption, at Rabaul in northeastern Papua New Guinea. The Talili Pyroclastics deposits are essentially a bimodal (dacite-basalt) tephra sequence that was emplaced during the 2.8 ky leading to the Rabaul Pyroclastics eruption. The dominant activity during the Talili period, starting at 4200 BP, involved strong (up to VEI 5), water-modified explosive eruptions of dacite from intra-caldera vents. This activity generated > 4.4 km3 of unstratified, bedded and laminated fine-grained fall deposits, pyroclastic surges and a small ignimbrite. In addition, all three of the young stratovolcanoes on the eastern and northeastern fringes of Rabaul Caldera were active during the Talili era, producing > 0.7 km3 of basaltic scoria fall and flow deposits. The main phase of Talili era basalt production occurred at 4100 BP and involved sequential eruptions from two of the stratovolcanoes, Palangiangia and Kabiu. The sequence of dacitic Talili eruptives shows a progressive decline then late-stage increase in SiO2 contents, with corresponding changes in Fe2O3, MgO and CaO. Early and late products have compositions similar to those of the most-evolved products of the major eruptions at Rabaul (intermediate to high-SiO2 dacites), while middle-stage products are similar to those of the historical eruptions (lower-SiO2 dacites and andesites). The basaltic eruptives have three different compositions, attributed to three different (stratovolcano) sources. There is no evidence of mixing or mingling of basalt and dacite. However, andesitic scoria inclusions in the early-erupted dacitic ignimbrite represent mingling of andesitic and dacitic magmas. As with the Talili Pyroclastics, at least two other earlier tephra sequences at Rabaul probably resulted from phases of VEI 5 volcanism and directly underlie the deposits of major eruptions. This leads to the inference that cycles of escalating eruptive activity have prevailed prior to some of the major eruptions, and suggests that the strength of volcanism at Rabaul may rise from the lower levels (maximum of VEI 4) evident since the Rabaul Pyroclastics event as the system moves towards the next major eruption.